This section is intended to introduce various aspects of the art that may be topically associated with embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Wellbores, for example, to reach hydrocarbon resources, are drilled using long pipe segments, which are joined together to form a drillstring. The drillstring typically terminates at a lower end with a drill bit (“bit”), which may be rotated from the surface by rotating the drillstring. Consequently, portions of the drill string may be in tension while other portions are in compression. A major portion of the drillstring is generally suspended in the borehole in tension, with a controlled amount of weight actually applied to the bit by portions of the drillstring itself. The actual weight-on-bit (WOB) may often be provided to the bit by one or more drill collars attached to the lower end of the drillstring above the bit. The WOB is applied by “slacking off” the tensioners on the rig floor. Drill collars are typically thick walled and provide more buckling resistance when applying WOB. Accordingly, the neutral point on the drillstring is often set to the point at which the drill collars start. Rotation of the bit through the drillstring, combined with a WOB, causes the bit to penetrate subsurface formations, as measured by a rate-of-penetration (ROP).
During drilling, the drillstring may buckle. Buckling is a deformation of the drillstring caused by the drillstring not being able to support an imposed compressive force. Buckling of the drillstring may be sinusoidal or helical. In either case, the drillstring touches the walls of the boreholes, creating additional friction and increasing the amount of torque required to turn the drillstring. The amount of force communicated to the drill bit, e.g., the WOB, also decreases, which may lower the efficacy of the drilling operation, e.g., as measured by the ROP. In the case of sinusoidal buckling, the drillstring touches the walls of the borehole at a number of points along the borehole, but the drillstring may still be rotated, depending on the amount of torque available from the drilling rig at the surface. In contrast, helical buckling places an entire section of the drillstring in contact with the wellbore, which will often increase the torque to levels that are too high for continued drilling. Further, helical buckling can lead to lock up, in which the drillstring cannot be turned or removed from the borehole.
Models have been developed to identify conditions that are likely to lead to buckling. For example, in Dawson, R. and Paslay, P. R., “Drillpipe Buckling in Inclined Holes”, SPE Paper 11, 167, September 1982, the authors present an equation used for buckling prediction of drill pipe in an inclined hole. These equations have been used to develop software to generate buckling threshold limits of a drillstring assembly at every point along the well path.
Further, techniques have been developed to model forces placed on a drillstring during operations. Johancsik et al., “Torque and Drag in Directional Wells—Prediction and Measurement,” IADC/SPE Paper 11,380, February 1983, present a mathematical technique to predict torque and drag in a wellbore. A computer program developed from the theory is also discussed. The equations for the modeling are presented in differential form in Sheppard et al., “Designing Well Paths to Reduce Drag and Torque,” SPE Paper 15,463, October 1986. Sheppard also discusses the concept of using simpler wellbore trajectories to reduce torque and drag. This is discussed further in U.S. Pat. No. 4,760,735, to Sheppard, which discloses a “method and apparatus for investigating drag and torque loss in the drilling process.”
A parameter called effective tension is discussed in Paslay, P. R., “Stress Analysis of Drillstrings,” SPE 27,976, August 1994. The effective tension is related to the true tension by accounting for the internal and external pressure on the drillstring, caused by fluid flow through and around the drillstring. The paper indicates that the effective tension may be used to determine if the string is buckled. If the drillstring is buckled, various techniques may be used to determine the decrease in the WOB caused by the transfer of part of that weight to the walls of the wellbore. See, for example, Wu, J., “Slack-off Load Transmission in Horizontal and Inclined Wells,” SPE paper 29,496, April 1995 (which presents a model to calculate the load (weight-on-bit) transfer inside a sinusoidally or helically buckled drillpipe).
Related information may be found in U.S. Patent Application Publication Nos. 2010/0133008; 2009/0157319; 2009/0319241; 2010/0185395; 2010/0307742; and 2011/005262. Further related information may be found in International Patent Publication Nos. WO 2010/101473 and WO 2011/005262. Other references which may contain related information are U.S. Pat. Nos. 4,792,703; 6,450,259; 6,612,382; 6,662,110; 7,261,167; 7,357,196; 7,730,967; and 7,556,104. Non patent literature references which may be related include Kenneth Bhalla, et al., “The Effect of Fluid Flow on Coiled-Tubing Reach,” SPE Production and Facilities, February 1998, pp. 59-63; and Robert F. Mitchell, “Tubing Buckling—The State of the Art,” SPE 104,267, December 2008.
Many of the techniques described in the references above may be useful for designing a drillstring and decreasing the likelihood of buckling in an enclosing annulus, such as in a borehole being prepared for use as a wellbore. The forces on the drillstring consist at least in part of tension/compression and torsional forces that are calculated by summing the individual segments of torque and drag generated at the bottom of the string all the way up to the surface. These “bottom-up” calculation techniques must assume a downhole weight-on-bit (DWOB) value and then back-calculate to the surface or drilling rig to determine what the hookload must be to obtain such DWOB. However, such techniques do not strictly conform to what is actually known on the rig surface during drilling operations, wherein one can readily measure the surface weight-on-bit (SWOB) and unless a downhole WOB measurement tool is used in the drill string, the actual WOB is only inferred and not confidently known. A method is needed that relies upon the known surface WOB (hook-load set-off) to accurately calculate at least one of the DWOB, a drill strong tension profile with WOB, and/or a drillstring rotating torque profile, by starting the determination from the top of the drillstring and marching or proceeding by calculation down to the bit, along each of the drill string compositional elements.